In the measurement of accelerated corrosion rate as disclosed in my U.S. Pat. No. 3,694,324, the corrosion current i.sub.A occurring at the free electrode potential is measured first.
When current i.sub.A is measured according to my U.S. Pat. No. 3,069,332, two measured electrodes are required, usually in the form of duplicated electrodes. The polarizing DC voltage is applied to these electrodes to produce the polarizing current i.sub.p which polarizes the one electrode cathodically by the voltage e.sub.pc and the other electrode anodically by the voltage e.sub.pa. When the ionic conductor resistance is negligible, the DC voltage applied to the electrodes measures the sum of e.sub.pc and e.sub.pa. Then, according to my U.S. Pat. No. 3,156,631, the measured current .sub.p is converted to the corrosion current .sub.A through linear proportionality with the Direct Voltage E.sub.d = 0.028 to 0.030 volt, as i.sub.A = (i.sub.p)(E.sub.d)/(e.sub.pa +e.sub.pc) = (i.sub.p)(E.sub.d)/(Applied DC Voltage). A second measurement of i.sub.A can be made with reversed polarity of the applied DC voltage, to average the two measurements.
Following measurements of current i.sub.A, the presence or absence of accelerated corrosion is detected by applying an increment of cathodic polarizing current i.sub.x to each of the duplicated measured electrodes, through means not interfering with corrosion current measurement, while measuring polarizing current i.sub.p. If current increment i.sub.x produces an increase in current i.sub.p, there is no evidence of the operation of accelerated corrosion mechanism, and the rate-determining corrosion current i.sub.R is determined as, i.sub.R = i.sub.A, where i.sub.R can be applied directly through Faraday's Law of Electrolysis to convert current into rate of metal weight loss.
If current increment i.sub.x produces a decrease in current i.sub.p, the presence of accelerated corrosion mechanism is indicated. Measurement is then made of change in value of current i.sub.p produced by increase in value of current i.sub.x, to determine the minimum value of current i.sub.p, termed i.sub.pb, produced by the application of current i.sub.x. Current i.sub.pb is converted to bounding corrosion current i.sub.B through the Direct Voltage E.sub.d, as described above. The accelerated corrosion mechanism then measures corrosion rate as, i.sub.R = 2.4(i.sub.A) -i.sub.B. Alternatively, but generally with less accuracy, the current i.sub.xb at which i.sub.pb occurs, can be measured and taken as i.sub.R = i.sub.xb.
The means for cathodically polarizing each of the duplicated electrodes by current i.sub.x while not interfering with the corrosion current measurement, is through what is termed a circuit isolation device, operated as follows. The positive pole of a source of variable DC voltage is connected to an additional electrode operated as an anode. The negative pole is connected to two isolation resistors of equal ohmic value. Each of the two duplicated electrodes is connected to the negative pole through one of the isolation resistors. The ohmic value of the isolation resistors is selected to be large enough to cause an acceptable small current to flow through them from DC voltage subsequently applied or produced between the duplicated electrodes during i.sub.p measurement. This current through the isolation resistors may be taken as a maximum value of about 10% of measured corrosion current i.sub.A. A meter in series with said source of variable DC voltage measures the total cathodic polarizing current 2i.sub.x.
In one alternative of said U.S. Pat. No. 3,694,324 the relationship between cathodic polarizing current i.sub.x and polarizing current i.sub.p of corrosion current measurement is continuously measured as current 2i.sub.x is applied at a selected rate of increase, but such measurement was not always reliable when operated through isolation resistors of equal value. Method and device improvement making such continuous measurement reliable is disclosed in my application Ser. No. 530,870. Circuit means is introduced to determine difference in free electrode potential. The range through which current 2i.sub.x is to be applied is determined from preceding measurement of corrosion current i.sub.A. Current 2i.sub.x is then applied at a selected rate of increase starting from zero, during which isolation resistor ratio is adjusted to maintain said determined difference in free electrode potential. Current 2i.sub.x is then removed, and a time lapse is allowed for substantial recovery to said difference in free electrode potential. A corrosion current i.sub.A ' is then measured with the isolation resistors connected but at 2i.sub.x = 0. The relationship between i.sub.p and i.sub.x is then continuously measured.
It is recognized that a desirable form of corrosion rate measurement device would include all measurement means and methods developed to date, with optimization made through a selection from the alternatives that they include. Optimization is directed to the relative merits of measurements made on the alternative electrode systems of one measured electrode with a reference electrode, or two measured electrodes. Measurement accuracy is further improved by extending the means for IR loss correction beyond that of ionic conduction between duplicated electrodes, to include the IR losses of the means for determining difference in free electrode potential and of the lead wires connecting the electrodes to the measurement device.